1. Field of the Invention
The present invention relates generally to a system for serial load leveling, and more particularly to a load leveling system for avoidance of compounded emissions in high energy, parallel, or distributed, pulse-modulated power control systems.
2. Description of the Related Art
Avoidance of the effects of simultaneous or coincidental switching of high currents is a challenge for systems having multiple pulse-width modulation (PWM) power switching circuits that share common supplies, enclosures, and other components. Part of the concern is the conducted and radiated EMI emissions resulting from compounded switching transients, but electrical and mechanical effects on the power source are also important.
Such a PWM power switching circuit is disclosed in U.S. patent application Ser. No. 13/479,105, filed on May 23, 2012, which is incorporated herein by reference.
Synchronized switching is preferred in such systems for reasons of determinacy. However, the conducted and radiated EMI effects of switching are compounded when multiple switches change at the same time. For example, FIG. 1 illustrates how simultaneously opening and closing multiple PWM circuits increases emissions far over that of a single PWM load. An additional effect may be observed as multiple synchronized PWM switches potentially combine to cause cumulative waveforms possessing strong fundamental frequencies significantly different than the fundamental frequency of the individual PWM switches.
As such, it is valuable to consider methods that provide determinant switching but reduce emissions, particularly those associated with simultaneous or compounded switching. This can be particularly important on aircraft where certain frequencies must be avoided for the safety of the vehicle.
Existing systems commonly generally distribute the switching events to reduce simultaneous switching. However, these methods retain a wide range of dynamic emissions, a significant portion of which have harmonic characteristics stronger than desired. It is very difficult to determine which of emissions these systems will experience at any moment in operation, so worst case emissions must be accounted.
Given a system with N switches and M possible pulse widths, using current phase spreading techniques, the range of emissions to characterize is on the order of NM. With Serial Load Leveling, the range of emissions with the technique is M, and those emissions are the minimum possible.
There are existing no-spread synchronized switching techniques that are fairly common. In Non-Spread Synchronized Switching systems, PWM switches are synchronized, share a common clock, and turn on simultaneously, but each one turns off separately according to individual duty cycles. An example of such a system is shown in FIG. 2. In a system supplied by a rotating generator, such power pulses have the effect of oscillating torque loads. In such systems, the maximum amplitude of these oscillations is the sum of the loads, e.g., having 4 to 16 loads yields a torque modulation 4-16 times that of a single load. For solid-state power supplies, the oscillating load causes analogous oscillating effects on the filtering and regulating components of the supply. The supply and other system components must be made to be robust to these oscillations, including avoidance of resonance with any significant harmonics.
There are also existing phase spreading techniques. A few methods exist to mitigate the effects of simultaneous switching loads. In general, they distribute or spread the individual PWM phases over the PWM cycle. However, they suffer either from lack of determinacy required for absolute characterization or from retention of significant transient or harmonic effects.
One such example is non-coherent phase spreading. One method of phase spreading is to introduce pseudorandom or non-coherent phase spreading to the PWM switch timing. This may be achieved by such means as random scattering of switch phasing or, more commonly, by reliance on presumed frequency drift of multiple non-coherent (wild) clocks. However, a lack of coherency complicates absolute system verification, that is, verification becomes a statistical exercise. Neither does non-coherent phase spreading eliminate the possibility of transient peaks, sustained problematic waveforms, or resonance.
Phase spreading has additional limitations in AC applications. Whereas phase spreading may have a relatively continuous switching distribution in DC applications, zero-cross AC switching quantizes the distribution—zero-cross AC switching imposes a common clock on all switches, regardless of any spreading.
Heretofore there has not been available a load leveling system or method for a power system with the benefits of the present invention.